Brake power boosters are known to utilize hydraulic fluid power, pneumatic power or a vacuum and atmospheric pressure differential to provide power assist in applying the brake system's master cylinder. Upon brake application, force on the brake pedal is transferred through the push rod to the power booster, which intensifies the force and transfers it to a type of piston that moves to actuate the master cylinder and apply the brakes. Brake power boosters are known to include a mechanism for transferring the feel of brake operation back from the master cylinder through the power booster to the brake pedal. A typical mechanism includes a reaction body and a reaction disc that are carried in the power piston transmitting forces between the power piston and the piston rod. When the brakes are applied, an apply force is developed to generate fluid pressure at the master cylinder, and that force is offset by being transmitted back through the piston, reaction disc and reaction body. The reaction disc is resilient so that it can be compressed and operates to transfer a portion of the total output force back through a reaction piston, and the pushrod to the brake pedal. This portion of the force, compared to the total force, is the reaction ratio.
With a conventional power booster, the amount of reaction force transferred to the push rod is a straight line function of input force until the unit reaches run-out. This means that regardless of the existing force level, in response to each additional incremental increase of 1 unit of output force from the power booster, an amount equal to x % of that 1 unit of output force is supported by the vehicle's driver at the brake pedal manually. For example, if the reaction ratio of the power booster is 2:1, then 2 units of output force results in 1 units of force being supported by the driver. Likewise, 20 units of output force results in 10 units of force being supported by the driver. To ensure that the input force requirements of a typical system are at an acceptable level over the entire operating range of the unit, the reaction ratio is selected for each individual application. If a harder pedal feel is preferred, then a lower reaction ratio may be selected. If a softer pedal feel is preferred, then a larger reaction ratio may be selected. A drawback is that once the reaction ratio is selected, it is applicable regardless of the actuation conditions that exist during operation of the vehicle's brake apply system.